Optical fibers are well known in the art and useful for many applications, including transmission laser devices and amplifiers. Basically, an optical fiber comprises an inner core fabricated from a dielectric material having a certain index of refraction and a cladding surrounding the core. The cladding is comprised of a material having a lower index of refraction than the core. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and it is guided along the length of the core. In most practical applications, the refractive indices of the core and cladding differ from each other by less than a few percent.
Designs for optical fibers vary depending upon the application, the desired mode of transmission of the light beam, or the materials used in fabrication. Fibers can be fabricated to propagate light with a single mode or multiple modes; an optical fiber which supports more than one guided mode is referred to as a multi-mode fiber. Multi-mode fibers typically have a larger core diameter than single-mode fibers to enable a larger number of modes to pass through the fiber. Additional design constraints are posed by multi-mode applications. Such constraints may include, for example, the choice of materials used to fabricate the core and cladding and the refractive index profile of the fiber (e.g., the profile reflects the radial variation in refractive index from the center of the fiber to the outer circumference of the cladding). Various types of refractive index profiles are known, e.g., step index, graded index, depressed clad, or W-type variety.
For high bandwidth, the group velocities of the various modes in multi-mode fibers should be as close to equal as possible. The differential group velocities can be controlled by grading the refractive index of the material comprising the core, which means specifying a functional form of the index as a function of the fiber radius. In a conventional multi-mode fiber, the design goal has been to achieve an .alpha.-profile, which is defined as: ##EQU1##
where r is the radius of the fiber, r.sub.core is the radius of the core, n.sub.clad is the refractive index of the cladding, and .alpha. and .DELTA. are free parameters. The optimal choice of parameters depends on the properties of the materials comprising the fiber and the intended application.
There are inherent limitations, however, in the .alpha.-class profiles, and manufacturing variables make it difficult, in practice, to achieve the theoretically optimal .alpha.-profile. For example, an inherent limitation with .alpha.-class profiles is that high order modes are not properly compensated (high order modes are those belonging to principle mode groups of high order; principle mode groups are groups of modes which propagate with nearly equal phase velocity, and the high order mode groups are those nearest to cutoff). Additionally, manufacturing variations can occur anywhere in the profile. With the two most frequently-used fabrication techniques, OVD and MCVD, anomalies are particularly problematic near the center of the fiber, i.e., anomalies occur near the center with greater frequency and magnitude than at other regions of the fiber. A common side effect of the MCVD process is a pronounced index depression, or dip, in the center of the fiber, which results in poorly compensated low order modes (i.e. those with small principle mode number). In other words, when a center dip is present the modes which have fields confined near to the central axis of the fiber have substantially different group velocities than the majority of the modes. Poorly compensated low order modes can dramatically affect fiber performance for certain applications, e.g. under launch from a semiconductor laser.
Efforts have been made to develop fiber index profiles to equalize high order modes in a multi-mode fiber and to compensate for the center dip. See Okamoto et al., "Computer-Aided Synthesis of the Optimum Refractive-Index Profile for a Multi-Mode Fiber," IEEE TRANS. MICROWAVE THEORY AND TECHNIQUES, Vol. MTT-25, No. 3 (March 1977), at p. 213 (incorporated herein) (hereinafter "Okamoto"). In Okamoto, a computer-aided synthesis is applied to develop an optimal profile, which is reported to be a smoothed W-shaped profile (e.g., FIG. 1 thereof). Essentially, this profile involves an extension of the alpha shape below the cladding (e.g., outside the core/cladding boundary region), with a negative cladding jump and then a further numerical refinement of the shape of the profile. See also Okamoto et al., "Analysis of Wave Propagation in Optical Fibers Having Core with .alpha.-Power Refractive-Index Distribution and Uniform Cladding," IEEE TRANS. MICROWAVE THEORY AND TECHNIQUES, Vol. MTT-24, No. 7 (July 1976), at p. 416 (incorporated herein), discussing use of numerical analysis to propose a similar profile. While such profiles may be advantageous in leading to high bandwidths, they are difficult to manufacture, and it is believed they may lead to leaky modes. In Geshiro et al., "Truncated Parabolic-Index Fiber with Minimum Mode Dispersion," IEEE TRANS. MICROWAVE THEORY AND TECHNIQUES, Vol. MTT-26, No. 2 (February 1978), at p. 115 (incorporated herein), a parabolic index profile is combined with a cladding jump, which leads to higher bandwidths than with a parabolic profile with no cladding jump. In a parabolic index profile, the core has a refractive index profile that has a parabolic distribution and is surrounded by a cladding having a constant refractive index.
As may be appreciated, those concerned with the development of optical communications systems continually search for new components and designs including new fiber designs. As optical communications systems become more advanced, there is growing interest in multi-mode fibers and increased fiber performance. The instant invention provides a multi-mode fiber having a refractive index profile that is relatively easy to manufacture and yet improves the behavior of high-order modes propagated by the fiber and compensates for the presence of a center dip, thereby improving the behavior of low order modes. Further advantages may appear more fully upon considering the description given below.